The invention relates generally to optical data distribution networks, and more particularly to a network including a plurality of drops and means for distributing optical data signals to the plurality of drops with each drop having listen-while-talk capability.
An optical data distribution network may include a plurality of drops and apparatus like a transmissive or reflective star, for example, for distributing optical data signals to the plurality of drops. An exemplary distribution network having a star configuration is shown in the block diagram illustration of FIG. 1. The optical signal distribution apparatus for this example includes an N port reflective star 10. A plurality of drops D1 through DN are coupled to the star 10 in such a manner as to permit any drop Di to communicate with any other Dj. Each drop, as exemplified by drop D1, for example, may include a transceiver 12 for transmitting and receiving optical signals. The transceiver 12 may be coupled to an optical signal directional coupler 14 which, in turn, may be coupled to the distributing apparatus 10 with a single length of optical fiber 16 for propagating transmit and receive optical signals between the directional coupler and the distributing apparatus 10. Each drop may further include optical connectors 18 and 20 for coupling the length of optical fiber 16 between the distributing apparatus 10 and directional coupler 14.
In operation, the optical directional coupler 14 of a drop is operative to direct the transmit optical signals from its transceiver unit 12 via an optical fiber line 22, for example, to the single length of optical fiber 16 thereof and to direct the receive optical signals from the fiber 16 to its transceiver unit 12 via an optical fiber line 24, for example. In the present embodiment, the reflective star 10 passively distributes the optical signals from one drop to the other drops. Some of the drops may include a drop interface unit, like that shown at 26 in drop D1, to permit communications with other optical networks or external equipment.
In some networks, there may be imposed a requirement that a drop be able to listen to its own transmission, i.e. have listen-while-talk capability. This capability may be used to provide assurance that the drop is operating properly. Theoretically, a network like that described in connection with the embodiment of FIG. 1, for example, should be able to meet this requirement. That is, the transmitted optical signal of a drop reflects from the star 10 and propagates back through its own single length of optical fiber 16 in the opposite direction and is split to a receiver in the transceiver unit 12 via directional coupler 14 and line 24, for example. However, in an actual installation, the various optical components of a drop may be connected together using optical connectors like 18 and 20, which may be imperfect. A typical optical connector may have an insertion loss of 1 db which means that about 80% of the power is transmitted through and 20% may be lost or reflected back. Accordingly, the reflected optical signal from an optical connector may cause an undesirable interference with the reflected signal from the star 10.
More specifically, referring to FIG. 1, the reflected component of the transmit signal from connector 18, for example, propagates back through the directional coupler 14 to the transceiver unit 12. In addition, components of the transmit signal, which travel through the length of fiber 16, are also reflected back from the optical connector 20 and from the star 10 through the optical fiber 16 and the directional coupler 14 to the transceiver unit 12. Moreover, all of the aforementioned reflected components of the transmit signal are received at the transceiver 12 delayed in time from each other because of the propagation time of light, i.e., the varying traveling distances of optical signals through the optical fiber 16. Accordingly, the resulting composite reflected transmit signal observed by the transceiver unit 12 may be badly distorted and unusuable as an accurate indication of the actual signal being transmitted thereby.
From the foregoing, it is apparent that, in practice, optical data distribution networks having drop configurations of the aforementioned type are generally not effective to achieve the desired listen-while-talk capability. Accordingly, applicants' inventive aspects are intended for the purpose of avoiding the foregoing described for each drop of the network, especially with little increase in cost per drop.